Antennas are physical structures that, when energized with electric signals having certain characteristics, generate electromagnetic waves that are emitted into the surrounding medium. Most antennas are designed to operate in free space (the Earth's atmosphere) to transmit the electromagnetic waves through the air. The air is a low loss environment, and radiation patterns having penetration depths of tens, hundreds, or thousands of times the length of the antenna can be achieved. Such antennas are not designed to operate in highly lossy environments, such as under the surface of the Earth.